[5] mqtt,mosquitto,eclipse Paho---MQTT Message Format analysis overview

Source: Internet
Author: User

We know that MQTT is a specification of an IoT protocol, and thelatest two versions of the MQTT protocol are: 3.1.1 and 3.1.0

(1) The specifications of 3.1.0 are as follows
Http://public.dhe.ibm.com/software/dw/webservices/ws-mqtt/mqtt-v3r1.html

(2) The specifications of 3.1.1 are as follows
http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718029

Normative 3.1.0 and 3.1.1 still have a lot of difference. For example, in the variable head, in the 3.1.0 specification, the keyword is, "MQISDP"

Table 1. MQTT V3.1 Specification The first 8 bytes of a mutable message header

  Description 7 6 5 4 3 2 1 0
Protocol Name
BYTE 1 Length MSB (0) 0 0 0 0 0 0 0 0
byte 2 0 0 0 0 0 1 1 0
byte 3 ' M ' 0 1 0 0 1 1 0 1
byte 4 ' Q ' 0 1 0 1 0 0 0 1
byte 5 ' I ' 0 1 0 0 1 0 0 1
byte 6 ' s ' 0 1 1 1 0 0 1 1
byte 7 d ' 0 1 1 0 0 1 0 0
BYTE 8 ' P ' 0 1 1 1 0 0 0 0

Table 2. MQTT V3.1.1 The first 6 bytes of a variable message header


Describe

7

6

5

4

3

2

1

0

Agreement name

BYTE 1

Length MSB (0)

0

0

0

0

0

0

0

0

BYTE 2

Length LSB (4)

0

0

0

0

0

1

0

0

BYTE 3

M

0

1

0

0

1

1

0

1

BYTE 4

' Q '

0

1

0

1

0

0

0

1

BYTE 5

T

0

1

0

1

0

1

0

0

BYTE 6

T

0

1

0

1

0

1

0

0

Protocol level

Describe

7

6

5

4

3

2

1

0

BYTE 7

Level (4)

0

0

0

0

0

1

0

0


The specification mainly describes the general format of the message, the command of the message, the flow of messages, and so on. In the previous four chapters, we have a perceptual understanding of MQTT, and in the next series of articles, I will take you to the following 14 kinds of command messages for MQTT through the capture package software (WireShark) and combine the specific specifications with everyone else. Note that because I use the Java Library of the Eclipse Paho API to support the version of Mqtt 3.1.1, and Mosquitto also supports the 3.1.1 version, all of the capture packages in this series will be based on the Mqtt 3.1.1 specification, not the MQTT 3.1 specification. Please pay attention to them. The following table describes the different message types in Mqtt 14.

Table 3. 14 Message types for MQTT (Command message)

Name

Value

Direction of information flow

Describe

Reserved

0

Forbidden

Reserved

CONNECT

1

Client to Server

Client request to connect to Server

CONNACK

2

Server to Client

Connect Acknowledgment

PUBLISH

3

Client to Server

Or

Server to Client

Publish message

Puback

4

Client to Server

Or

Server to Client

Publish Acknowledgment

Pubrec

5

Client to Server

Or

Server to Client

Publish received (assured Delivery Part 1)

Pubrel

6

Client to Server

Or

Server to Client

Publish release (assured Delivery Part 2)

Pubcomp

7

Client to Server

Or

Server to Client

Publish complete (Assured Delivery Part 3)

SUBSCRIBE

8

Client to Server

Client Subscribe Request

SUBACK

9

Server to Client

Subscribe Acknowledgment

Unsubscribe

10

Client to Server

Unsubscribe Request

Unsuback

11

Server to Client

Unsubscribe Acknowledgment

PINGREQ

12

Client to Server

PING Request

Pingresp

13

Server to Client

PING response

DISCONNECT

14

Client to Server

Client is disconnecting

Reserved

15

Forbidden

Reserved


In the following chapters we will be one to everyone with Wireshark grab bag, and give everyone an analysis of their agreement, please look forward to and attention, thank you.

[5] mqtt,mosquitto,eclipse Paho---MQTT Message Format analysis overview

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